Method and apparatus for determining the oxygen demand of oxidizable materials



V. A. STENGER ET AL METHOD AND APPARATUS FOR DETERMINING THE OXYGEN 03,421,856 EMAND 0F Jan. 14. 1969 Sheet OXIDIZABLE MATERIALS Filed Dec.29, 1965 @N NM M INVENTORS. Vernon 14. S/enger BY C/ayfon E. Van f/o//F) 77 ORA 15' V Jan. 14. 1969 METHOD AND APPARATUS FOR DETERMINING THEOXYGEN DEMAND OF Filed Dec. 29, 1965 v'. A. STENGER AL OXIDIZABLEMATERIALS Row 6ewc79e 5 9 50mp/e numbers Sheet 2 012 Aaw sewa freaiec/and 6///o Zhns.

Samp/e numbers INVENTORS.

Verna/7 H. Jfe/v er C/ay/onf l ar) Ha QTTOANEY United States Patent O3,421,856 METHOD AND APPARATUS FOR DETERMINING THE OXYGEN DEMAND OFOXIDIZABLE MA- TERIALS Vernon A. Stenger and Clayton E. Vanliall,Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich, acorporation of Delaware Filed Dec. 29, 1965, Ser. No. 517,298 U.S. Cl.23230 Int. Cl. Gllln 31/00 The present invention relates to the problemof analyzing combustible or oxidizable materials for their oxyen demand.A special and most useful embodiment of the invention involvesdetermining the total oxygen demand (TOD) of aqueous systems, e.g.,waste streams. The invention is particularly concerned with a method andapparatus for measuring the TOD of oxidizable materials.

In the field of sewage treatment, chemical oxygen demand (COD) has longbeen used as a measure of pollution. A common method for determining CODis de scribed in a publication of the mnerican Public HealthAssociation, Standard Methods for the Examination of Water and WasteWaters, 11th edition, New York, 1960, page 399. Basically, the techniquedescribed therein involves oxidation of the sewage sample with potassiumdichromate in 50 percent sulfuric acid. The amount of dichromate reactedreflects the extent of oxidation, thus titration of residual dichrom-ateyields a measure of the oxygen demand of the system. Although thismethod yields useful results, the length of time to achieve an analysisis excessive.

It would be desirable, and it is a principal object of the presentinvention to provide a rapid method for measuring the oxygen demand ofan aqueous system. A further and more comprehensive purpose of theinvention is to provide a convenient method for measuring the oxygendemand of any oxidizable material. An additional object is to provideapparatus for carrying out the foregoing determinations. Still furtherobjects are to provide methods and apparatus which increase thesensitivity of the measurement. These objects, and other benefits. aswill become apparent hereinafter, are accomplished in accordance withthe present invention.

Accordingly, the invention involves a method for detor-mining the oxygendemand of a material containing oxidizable components, which methodinvolves the combustion of a small sample of the material to be analyzedin a heated, continuous stream of carbon dioxide. As Will be shown inthe following examples, the carbon monoxide produced as the result ofcombustion with carbon dioxide relates directly to the total oxygendemand (TOD) of the sample. Hence, quantitative analysis of thecombustion gases for carbon monoxide yields a measure of the oxygendemand of the sample. As used here:'n, combustion refers to the reactionor equilibration of carbon dioxide with an oxidizable material in thesense that the oxidant (carbon dioxide) is reduced and the oxidizablematerial is oxidized. By TOD is meant the net oxygen demand of thesample. Thus oxygen dissolved in the sample and other oxidant sourcematerials contained in the sample lower the oxygen demand of the sampleas it is measured in accordance with the invention.

In process terms, the invention involves establishing a flowing feed gasstream containing carbon dioxide as essentially the sole oxidant. Thisgas stream is passedthrough a combustion conduit having a heating zoneat 21 Claims a temperature high enough to cause some combustion, orequilibration, of the oxidizable components of the material to beanalyzed with carbon dioxide. Such equilibration occurs to a degree attemperatures as low as about 500 C., but preferred and more uniformcombustion is achieved at temperatures above about 650 C. The upper endof the temperature range is limited by the fusion temperature ofmaterials used in the heated zone of the combustion conduit, but it ispreferred that the temperature should not exceed 1000" C.

Contained within the heating zone of the combustion conduit is agas-permeable catalyst bed through which the carbon dioxidecontaininggas stream flows. The bed is preferably at least about 2 cm. in length.It is the function of the catalyst bed to promote the equilibration ofcarbon dioxide with oxidizable components of the material analyzed toproduce carbon monoxide.

Suitable catalyst materials include for example, high melting noblemetals such as platinum, palladium, iridium, rhodium, ruthenium andgold. Siliceous materials such as quartz are effective to a degree. Apreferred catalyst is platinum. For purposes of economy, the noblemetals are used in a form which presents a large surface area per unitweight of the metal. O ften such catalysts are coated on an inertsupport. To be suitable in general, a catalyst should be effectivelyfree from substances that reduce carbon dioxide to carbon monoxide, orsupply oxygen to organic matter in the sample. Thus, iron, nickel,copper and similar metals, which are reactive either with carbondioxide, carbon monoxide, oxygen, or components of the samples, shouldbe excluded from the high temperature zone. Similarly the higher oxidesof most elements should be excluded.

From the heating zone and catalyst bed the gas stream is passed into ananalyzer for quantitatively determining the amount of carbon monoxide inthe presence of carbon dioxide. Analytical devices for this purpose areknown. One particularly suitable for use in the present invention is anon-dispersive infrared analyzer which produces an electrical signal inproportion to carbon monoxide content of the gas stream. The signal maybe read-out by any convenient means such as a graphic recorder.

Having established a carbon-dioxide feed gas stream flowing through thecombustion conduit and thence into the carbon-monoxide analyzer, aquantity of the combustible material to be analyzed is inserted into theheated zone of the combustion conduit on the upstream side of thecatalyst bed. The continued flow of the gas stream sweeps gaseousproduct formed through the catalyst bed. Efiluent gas from the heatedzone containing the equilibrated gaseous reaction product of carbondioxide and oxidizable components of the material to be analyzed flowsinto the carbon-monoxide analyzer whereby the incremental increase incarbon monoxide of the gas stream is measured.

In the preferred practice of the invention this measure ment is obtainedin the form of an electrical signal which is a function of thecarbon-monoxide content of the effluent gas. Such a signal is readilycalibrated to provide a direct reading of carbon monoxide produced and,as will be demonstrated hereinafter, the total oxygen demand (TOD) ofthe sample analyzed.

It will be seen that .the feed gas serves simultaneously as the reagentwhich oxidizes reducing material in the sample, and as a carrier gaswhich sweeps the reaction products from the combustion zone and throughthe de- 3 tcctor. After the gas has left the combustion conduit it isreferred to as the etfiuent gas.

The feed gas containing carbon dioxide as essentially the sole oxidantmay also contain any one or more of the inert gases such as nitrogen,helium, argon, krypton and the like and ordinarily will contain somesmall amount of oxygen as an impurity.

Although a gas containing small amounts of oxidizing components can beused in the practice of the invention to provide useful results, greateraccuracy and sensitivity is obtained by assuring effective eliminationof oxygen and other gases of greater oxidation potential than carbondioxide by incorporating a volatile reducing component into the gas inan amount more than sufiicient to react with oxidizing impurities in thefeed gas. Any one of a number of reducing reagents such as hydrogen,carbon monoxide, methanol, acetone, or ammonia may be incorporated incontrolled amounts into the feed gas stream for this purpose. When thegas mixture is heated, the reducing component reacts with the oxidizingimpurity.

Best results are achieved in the following manner: Carbon dioxide or amixture thereof with an inert gas such as nitrogen or argon is passedthrough a heated, gas-permeable bed of carbon. By suitably adjusting thetemperature of the carbon bed a small portion of the carbon dioxide isreduced to carbon monoxide. Temperatures of the carbon bed usually fallwithin the range from about 450 to 560 C. Within the combustion zone,this carbon monoxide reacts with any oxygen or other oxidizingimpurities present. The atmosphere thus produced is, for the purposes ofthe invention, actually an oxidizing atmosphere in which carbon dioxideis effectively the sole oxidant.

With a given flow rate and composition of carbon dioxide-containing gas,the reaction of the gas in a carbon bed at a given temperature producesa feed gas with uniform or constant concentration of carbon monoxide.The presence of carbon monoxide in the feed gas at a constant low levelrelative to the carbon dioxide therein does not interfere with themeasurement of the incremental increase of carbon monoxide resultingfrom the reduction of carbon dioxide by the sample to be analyzed.

In another mode of operation, the reducing quality of the feed gas at asomewhat higher carbon monoxide level affords an opportunity to measurethe net oxidizing capacity (NOC) of materials analyzed rather than theirTOD. This occurs as the result of the generation of a negative signal.Such a signal is defined by a dip in the carbon monoxide concentrationof the effluent gas as the result of the oxidation of carbon monoxide byoxidizing components of the sample. Whether the same has an oxygendemand or an oxidizing capacity can be determined readily by observingwhether the carbon monoxide concentration of the efiluent gas isincreased or decreased upon injection of the sample. When the inventionis used in this manner, enough carbon monoxide is introduced into thefeed gas to fully reduce the oxidizing capacity of the sample. Usuallythe feed gas will contain at least about 0.05 volume percent carbonmonoxide. Normally, it will not exceed 1 volume percent carbon monoxide,but higher amounts may be used if desired.

Although it is not necessary, it is usually preferred to pass thegaseous effluent from the heating zone through a cooling zone whereinthe temperature of the gas stream is lowered to a temperature below thatof the apparatus used for detecting the carbon monoxide. Thus, moisture,if any is present, is largely separated from the gas stream prior to itsentry into the detector. Such condensate is accumulated in the coolingzone and means are provided for its collection and removal as needed.

As previously stated, the invention is applicable to the analysis of allmaterials containing oxidizable components. Thus, gases and solids asWell as liquids can be analyzed for their TOD in accordance with theinvention. Sample size is not critical, but small samples on the orderof 0.001 to 0.1 cubic centimeters for liquid and solid samples and 0.001to 5 cubic centimeters for gas samples permit the use of equipment ofconvenient design.

A major and most useful application of the invention is to the analysisof aqueous system containing oxidizable components. A special designconsideration important to the success of such analysis is thepositioning of the catalyst bed within the combustion conduit. For suchoperation, the catalyst bed is positioned within the heated zone of thecombustion conduit at some distance from the gas inlet. This distance issufiicient to define, in conjunction with the confines of the conduititself, a sample expansion zone within the heated zone. Upon injectionof the liquid sample to be analyzed, sample vapors generate for aninstant some back pressure. The feed gas within the sample expansionzone at the instant of injection forms a gas blanket which preventssignificant diffusion or back flow of sample vapors out of the heatedzone. Sufficient volume of the sample expansion zone is indicated by theabsence of condensate formation in the inlet of the feed gas stream.

For best operation in analyzing liquid materials, the line of sampleinjection should deposit a sample at about, or on, the upstream face ofthe catalyst bed. A line of injection which is essentially parallel tothe longitudinal axis of the combustion conduit and the application ofsufficient injection force assures such a result.

Various carbon monoxide detection techniques enable determination of thetotal amount of carbon monoxide formed according to the integral whereinQco is the amount of carbon monoxide generated upon injection of thetest sample and dq/dl is the differential of carbon monoxide in theeffiuent gas at any one instant. The time period of carbon monoxidevariation from the feed gas stream is defined by 1 -1 The preferred modeof operation involves correlating carbon monoxide content with somecharacteristic of an electrical signal generated by the carbon monoxidedetector. For example, an amperometric or potentiometric signal willexhibit displacement from a normal base line. The height or amplitude ofthe displacement can be correlated with the change in carbon monoxidecontent of the effluent gas. To achieve such operation, however, certainparameters of the process should be controlled to provide reproducibleresults. For instance, it is necessary that the feed gas stream becontrolled to a predetermined and constant rate of flow (predeterminedmeans preset level; knowledge of absolute flow rates is not necessary).It will be determined for any particular equipment design chosen, i.e.volume of combustion conduit, flow rate capacity of carbon monoxidedetector and temperature of the combustion conduit, that there will be arange of flow rates over which optimum signals will be generated. Thusfor equipment of a given design, an optimum flow rate is readilydetermined by measuring a sample with a known TOD over a series ofincrementally increasing flow rates. In this manner, an optimum flowrate will be defined which produces a sharp and discriminating signaland is preferably relatively insensitive to minor variations in flowrate. This technique will be illustrated with reference to a particularapparatus in the examples.

Normally with equipment of convenient design the combustion conduit willhave a bed volume within the range from about 10 to 200 cubiccentimeters, preferably from about 25 to about cubic centimeters. By bedvolume is meant the total volume of the heated zone within thecombustion conduit. For liquid samples, the sample size will usuallyrange from about 0.005 to about 0.5 percent, preferably 0.01 to 0.1percent. of the bed volume. Combustion temperatures are usually withinthe range of about 800' to about 900 C. Such temperatures promoteefiicient equilibration of carbon dioxide with oxidizable components ofthe test sample and signal characteristics subsequently generated byelectrical detectors of carbon monoxide are relatively independent ofsmall variations in the combustion temperature.

Detectors which may be used to measure the carbon monoxide in theefiluent gases from the combustion conduit include any of the knownmeans for quantitatively analyzing a gas stream for its carbon monoxidecontent. As previously mentioned, the preferred detector produces anelectrical signal, the strength of which can be correlated with theconcentration of the measured quantity. A preferred detector is anon-dispersive, infrared analyzer sensitized for carbon monoxide. Asignal output from such an analyzer is adapted by a suitable amplifierand graphic read-out means, such as a strip chart recorder, to providereadings which can be converted to, or read directly as, carbon monoxideconcentration in the effluent gas and hence the TOD of the test sample.To provide comparable analytical readings for calibration of thegenerated signal, care must be exercised to insure that test samplevolumes, amplifier gain, recorder voltage, and process operatingparameters involving temperature and gas flow rates are identical orwithin operational levels at which the analytical results areindependent of these variables.

Apparatus for carrying out the described analytical process, and certainpreferred embodiments thereof are illustrated in the accompanyingdrawings.

FIGURE 1 is a schematic drawing of a complete apparatus suitable foraccomplishing the analysis of liquid or gaseous materials containingoxidizable components. The figure also illustrates the optional carbonmonoxide generator.

FIGURE 2 is a detailed illustration of a combustion conduit containing acatalyst bed.

FIGURES 3 and 4 show comparative results of COD measurements made by theprior art technique and the TOD measurement of the present invention ontwo series of sewage samples.

The apparatus of FIGURE 1 comprises carbon dioxide feed gas supply means2, sample injection means 3, heating means 4, a combustion conduit 22within the heating means 4, cooling means 5, condensate removal means 6integral with the cooling means 5 and carbon monoxide detection means 7.In the illustrated preferred embodiment, the feed gas supply meanscomprises a carbon dioxide gas tank 11 which feeds carbon dioxidethrough a series arrangement of a pressure regulator 12, turnoff valve13, and flow meter 14. This series arrangement constitutes a gas flowcontrol means 8.

From the gas flow control means 8 the feed gas stream enters a carbonmonoxide generator 9. This consists of a furnace 19 having a heatingzone 17 in which there is positioned a conduit 18. Within the conduit18, is a gas-permeable carbon bed 20 held in place by gas-permeable,positioning elements 10, 21 at each end of the bed 20. Temperaturecontrol in the carbon monoxide generating means is obtained with thepower control 26. Temperature readings are obtained by means of apyrometer 27. Each end of the conduit 18 is provided with connectingmeans 16 for attachment with preceding and succeeding apparatuscomponents.

The feed gas stream flows from the carbon monoxide generator 9 through acheck valve 15 into the combustion heating means 4 comprising a furnace24 in which there is a combustion conduit 22 having a heating zone 25.Temperature control in the heating means is achieved through a powercontrol 30. The temperature is measured by means of a pyrometer 28. Atthe gas inlet end of the combustion tube 22 is an injection means 3,such as the illustrated syringe 23.

Gaseous effluent from the combustion conduit 22 passes into coolingmeans 5, which in the illustration is an aircooled condenser 29 equippedwith condensate removal means 6 in the form of a stopcock 31 fordischarge of accumulated condensate. The cooled effluent gases then passthrough a filter 32 to remove any solid particulate matter and thenceinto carbon monoxide-detection means 7.

The illustrated carbon monoxide detection means 7 consists of anelectrically connected combination of a non-dispersive, carbonmonoxide-sensitized, infrared analyzer 35. This analyzer produces avariable voltage signal to be amplified by means of a low-voltageamplifier 38. The enhanced electrical signal is fed into a continuousgraphic recorder 39 which produces a curve on a paper strip 42. Eitherthe amplitude of or the area under the curve 41 is a function of thecarbon monoxide in the effluent gases measured in the detection cell 36of the infrared analyzer 35. Useful controls in the detection means arethe amplifier gain control 43 and the recording voltage range control 44After passing through the detection cell 36 the efliuent gas may bedischarged to the atmosphere through a vent 37 or by means of a valve 33returned through optional gas-return means comprising a conduit 79 to agas blanket jacket surrounding a portion of the injection syringe 23.Gas returned in this manner provides a blanket of oxygen-free gasinsulating the injection port to the combustion conduit.

The various components of the foregoing apparatus are interconnected toprovide a continuous gas stream with suitable gas conveying conduits 69,70, 71, 72, 73, 74, 75, 76, 77 and 78.

In FIGURE 2 the combustion conduit 22 is shown in more detail. Itconsists of two separable parts which are a feed gas inlet 54 and acylindrical combustion tube 51. Seated within the feed gas inlet 54 isan injection tube 52 adapted to receive the syringe 23. The injectiontube 52 is aligned in a direction essentially parallel to thelongitudinal axis of the combustion tube 51. The syringe 23 issurrounded by a protective gas jacket 80. The feed gas inlet 54 iscoupled with the cylindrical combustion tube 51 through a ground glassjoint 53. Within the cylindrical combustion tube 51 is a catalyst bed 57of platinum gauze balls 61 maintained in place by means ofcatalyst-positioning elements 59 and 60, the latter of which is seatedagainst an indentation 63 in the tube 51. Each end of the assembledcombustion conduit 22 is adapted for coupling with preceding andsucceeding apparatus elements. The upstream feed gas inlet is a smalltubular nipple 58 and the downstream outlet, coupling means is a ballportion 62 of a ball joint.

Certain preferred embodiments of the above-described fundamentalapparatus components have been set forth. Numerous alternatives willoccur to those skilled in the art. Modifications necessary to adapt theapparatus for the analysis of solid and gaseous samples, as well asliquid samples, will readily occur to those skilled in the art.

For instance, with regard to the feed gas supply means 2, it is onlynecessary that there be provided a confined stream of carbon dioxidepreferably subject to precise flow rate control. With respect topreferred operation, knowledge of the actual flow rate is not necessaryso long as the gas flow rate can be controlled to a predetermined andconstant rate. To this end any combination of mechanical means forsupplying and regulating a gas stream can be uesd in place of thatillustrated. Insofar as heating means 4 is concerned, apparatus capableof providing controlled heating over a temperature range of 500 to 1000C. can be used. Although an electric resistance furnace is efiicient forthis purpose, induction heating means, or any other convenient heatingmeans, can be used.

Similarly, sample injection means 3 can be provided by any mechanicalapparatus capable of supplying measured aliquots of materials andinserting them into the heating zone 25 of the combustion conduit 22.For example, direct insertion of a liquid sample to be analyzed into theheating zone 25 can be accomplished by sprayers adapted to providecontrolled amounts of sample spray. Injection of solid samples isreadily achieved by known means. For instance, if the combustion tube 22is aligned vertically, the sample is simply dropped into the heatedzone.

If cooling of gaseous effluent is desired to protect or optimize thedetection operation, such cooling can be accomplished in a conventionalmanner such as by passing the gaseous efiluent through the illustratedaircooled condenser 29. Alternately, water-cooled condensers areeffective for this purpose. It is possible, however, to obtain usefulresults without cooling the gaseous effluent.

Although it is not necessary to operability, it is preferred to employ agas filter 32 which will separate any particles or moisture entrained inthe gaseous product prior to its introduction into the detection cell36. The particular carbon monoxide detection means 7 described above ispreferred but any analytical apparatus capable of indicating thequantity of carbon monoxide in the gaseous product with desiredsensitivity and specificity can be used. Illustratively, fuel cells andgalvanic sensing devices may be adapted for the analysis of carbonmonoxide.

Materials of construction employed in the above combustion gas trainmust generally meet the criteria of having resistance to carbon dioxideand moisture. Moreover, it is desirable, at least in the gaseous producttrain, that materials of construction be essentially nonreactive tocarbon monoxide. Within the combustion zone itself, it is necessary thatthe materials of construction be inert to the combustion products ofsamples analyzed at the elevated temperatures used for combustion. Suchmaterials include, for example, fused silica, Vycor glass, glazedceramics and the like siliceous materials.

In a specific embodiment of the above-described apparatus shown inFIGURE 1, inch stainless steel tubing was utilized to provide theconnecting conduits 69, 70 and 71, and 3 inch butyl rubber tubing wasutilized to provide the connecting conduits 72, 73, 74, 75, 76, 77, 78and 79. The carbon dioxide pressure regulator 12 was 21 Watts RegulatorType 26 Model M1 and the valve 13 consisted of a Hoke needle valve. Theflow rate was measured with 2. Brooks Flowmeter 14 Type 2l110 with anR-2-15AA tube and a stainless steel float.

The carbon generator 9 consisted of an electric mufile furnace 19operating on a voltage of 120 volts and a maximum power consumption of900 watts. The power control 16 was a Powerstat variable voltagetransformer. An Assembly Products, Inc. pyrometer 27 Model 4526 was usedto indicate the temperature.

A cylindrical tube 18 consisting of fused silica and having an insidediameter of 1.27 centimeters and a length of 40 centimeters was used inthe construction of the heated zone 17 of the carbon monoxide generator.Within the tube 18 at about 24 centimeters from the inlet end thereofwas placed a gas-permeable carbon bed 20 about 4 centimeters longconsisting of granulated cocoanut charcoal held in place by positioningelements 10 and 21 of quartz woo] one centimeter long at each end. Eachend of the carbon monoxide generator tube was provided with a ball jointas coupling means 16. A Kimble valve No. 38006 was used for the checkvalve 15.

Combustion-supporting temperatures within the combustion conduit 22 weregenerated with an electric muffie furnace 24 operating on a voltage of120 volts and a maximum power consumption of 900 watts. The powercontrol 30 was a Powerstat variable voltage transformer.

The combustion tube 51 was a fused silica cylinder having an insidediameter of 1.27 centimeters and a length of about 40 centimeters. Theheated zone 25 of the combustion conduit 22 was about centimeters long.A gas inlet 54 was provided in the form of a tubular glass T,

with the cross bar of the T having a Vycor ground glass joint 53 at oneend for coupling with the fused silica combustion tube 51 and a No. 18stainless steel syringe needle 52 about 4.8 centimeters long seated inthe opposite end of the cross bar as receiving means for sampleinjection means in the form of a syringe. When the components of thecombustion conduit 22 were assembled, the needle 52 was directed in aline essentially parallel with the longitudinal axis of the combustiontube 51. The stem of the tubular glass T provided a nipple 58 forconnection with the W inch gum rubber interconnecting conduit 74. AHamilton No. 705N syringe 23 was employed as the injection means 3.

Within the combustion tube 51 at about 24 centimeters from the inlet endthereof was placed a catalyst bed 57 about 13 centimeters longconstructed of platinum gauze balls 61. The catalyst bed was held inplace by catalyst positioning elements 59 and 60 in the form of quartzwool plugs one centimeter long on both ends of the platinum gauze balls.The catalyst bed was formed by gently tamping one quartz wool plug intoplace against a retaining indentation 63 within the combustion tube 51with a glass rod, adding the platinum gauze balls and then the secondquartz wool plug. After its component parts had been assembled, thecombustion conduit 22 was placed within the electric muffle furnace 24so that the tip of the syringe needle 52 was just outside the heatingzone of the furnace 24 but yet in position such that, upon injection ofthe aqueous sample, the full amount thereof was deposited within theheating zone 25 of the combustion conduit 22.

The gaseous products produced upon injection of a test sample wereconducted through a gas train consisting of a series arrangement of anair-cooled condenser 5, a U- shaped water trap 29 and a gas filter 32containing a 5-9 micron filtering element. The water trap 29 was adaptedfor intermittent drainage of accumulated water by means of a stopcock31. The interconnecting conduits 76 and 77 consisted of inch butylrubber tubing.

Carbon monoxide detection means 7 employed with the foregoing apparatusconsisted of an infrared analyzer 35 (Beckman Model 21A) equipped with a13.3 centimeter detection cell 36 sensitized for determination of carbonmonoxide. The detection cell 36 was maintained at a temperature of 45 C.to prevent the formation of condensate which would interfere with theaccuracy of the analytical result. Output from the analyzer 35 was fedby electrical leads 64 and 65 to a low voltage amplifier 38.Subsequently, the amplified output of the analyzer was fed into agraphic recorder 39 (Sargent Model MR) through electrical leads 66 and67. The recorder 39 was set by the voltage recording range control 44 tooperate in the 0-2.5 millivolt range. The gain control 43 of theamplifier 38 was set at a predetermined level to provide a desiredresponse in the recorder 39.

EXAMPLE 1 Using an apparatus such as that described above, a series ofoperations was carried out to demonstrate the utility of the describedmethod for determining the total oxygen demand of various materials.Most of these operations were carried out on aqueous dispersions ofoxidizable materials, but the process with certain modifications toaccommodate the materials handled, can be adapted to the analysis of anyoxidizable material whether it be gas, liquid or a solid.

To carry out these analytical operations, a fiow of carbon dioxide wasestablished through the apparatus at a rate of about cubic centimeters(S.T.P.) per minute. The carbon-monoxide generator was heated at atemperature of about 540 C. The reaction of carbon and carbon dioxideproduced carbon monoxide, which in turn reduced free oxygen present inthe carbon dioxide, leaving a small residual of unreacted carbonmonoxide in the feed gas. The heated zone of the combustion conduit wasbrought up and maintained at a temperature of about 875 C.

Initially a calibrating curve was prepared for converting signalamplitude, i.e., recorded curve peak height, into the total oxygendemand (TOD) of analyzed samples. To this end, solutions were made upcontaining varying amounts of sodium acetate to provide a series ofaqueous solutions with incrementally increasing known oxygen demands.

Subsequently, using the data obtained on sodium ace tate solutions as areference standard, a series of oxidizable materials representing bothorganic and inorganic species were analyzed in accordance with theinvention. The results of these experiments are set forth in thefollowing Table I.

.In an aqueous solution.

2 Calibrated with data obtained on solutions of sodium acetate withknown TOD values.

3 Calculated oxygen demand equals milligrams of oxygen required perliter of solution to give complete oxidation of the sample to water,carbon dioxide and nitrogen.

Ammonium chloride was the only material of those tested that yieldedresults higher than calculated. This is probably due to a secondaryreaction of hydrogen chloride with carbon dioxide in the presence ofplatinum. Samples of hydrochloric acid or sodium chloride containing thesame chloride concentration as the ammonium chloride standard gaveoxygen demand results that corresponded to the amount that the ammoniumchloride standard was high, thus substantiating the proposed cause ofthis interference. However, it has been found that chloride in thepresence of an oxidizable material such as sodium acetate exhibits lowerinterference.

EXAMPLE 2 The method of the invention has been applied to the analysisof waste waters. Two waste streams were tested. One stream was a rawsewage which had been clarified by settling for two hours. The otherstream was from the same raw sewage but had been treated with a highmolecular weight, anionic polymer flocculant and the resultingsuspension also settled for two hours. Twelve daily composite sampleswere taken from each stream.

Each of the composite samples was divided into two portions, one ofwhich was subjected to a conventional COD analysis by chemicaloxidation. The other was blended for 5 minutes in a Waring Blendor toproduce a uniform dispersion, which was then subjected to analysis inaccordance with the invention.

The results of these experiments are plotted in FIG- URES 3 and 4. Aclose correlation in the results of the two methods is evident.

EXAMPLE 3 The general applicability of the analytical process set forthherein is illustrated according to the following mathematical treatment.Assuming an oxidation reaction for the types of compounds most likely tooccur in domestic waste streams, one obtains a generalized equation asfollows:

Manifestly, to balance the above equation the value of n (the number ofoxygen atoms required) is:

Solving the equation for It requires the determination of threevariables. The value of a can be determine-d as the total carbonaccording to the method of patent application Ser. No. 380,597, now U.S.Patent No. 3,296,435. However, the values of band d are independentvariables not readily measured, particularly with a dilute aqueoussample. Thus, there exists neither a true correlation with total carbonnor a technique for the direct measurement of b and d in the aboveequation to provide a useful method of determining n, the oxygen demand.

In accordance with the invention, however, carbon dioxide gives up apart of its oxygen to produce an oxidation product and carbon monoxideas the reduction product of the carbon dioxide. Since carbon dioxide canonly oxidize carbon to carbon monoxide, the equation illustrating such acombustion or oxidation reaction is as follows:

To balance the above Equation 3 with respect to oxygen the followingequation must apply:

Solving for m by merely transposing terms:

The (m+a) the quantity of carbon monoxide which is measured, becomes:

By comparing Equation 6 with Equation 2, it will be seen that the valueof (m-l-a) is the same as the value of 11 wherein n is the oxygen demandexpressed in the number of atoms. In other words, the quantity of carbonmonoxide produced, in molecules (whether from the oxidation of carbon orthe reduction of carbon dioxide), is the same as the number of oxygenatoms that would be required for complete oxidation.

Consequently, by injecting the aqueous sample to be analyzed into aheated stream of carbon dioxide as in the above examples and passing thegases through an analyzer sensitized to determine carbon monoxide, oneis able to determine the total oxygen demand of the test sample. Ofcourse, if the sample also contains an oxidant, such oxidant detractsfrom the amount of oxygen that must be obtained from carbon dioxide.Nevertheless the value measured represents the net total oxygen demand.

If desired, the net oxidizing capacity of a sample can be measured as afunction of the drop in carbon monoxide in the effluent gas immediatelyafter injection of the sample. Such a measurement correlates with theoxidizing capacity of the sample analyzed and can be calibrated byreference to known standards in the same manner that peak heights abovethe base signal line can be calibrated to give TOD.

What is claimed is:

1. A method for determining the oxygen demand of a material whichcomprises:

flowing a feed gas stream containing carbon dioxide as essentially thesole oxidant into a combustion conduit having a heating zone at atemperature of at least 500 C. and through a catalyst bed in the heatingzone of the combustion conduit, said catalyst bed being effective topromote the equilibration of carbon dioxide with oxidizable componentsof the material to be analyzed,

inserting a predetermined quantity of the material to be analyzed intothe gas stream on the upstream side of the catalyst bed within thecombustion conduit, and

sweeping gaseous product formed through the catalyst bed and effluentgas from the heating zone into an analyzer for quantitatively indicatingcarbon monoxide.

2. A method as in claim 1 wherein the heating zone of the combustionconduit is maintained at a temperature within the range from 500 to 1000C.

3. A method as in claim 1 wherein the flowing feed gas stream isestablished at a constant and predetermined flow rate and the heatingzone of the combustion conduit is maintained at a temperature within therange from 500 to 1000 C.

4. A method as in claim 3 wherein the analyzer continuously monitors theeflluent gas to produce an electrical signal relative thereto andcalibrating such signal to determine the total oxygen demand of thematerial analyzed.

5. A method as in claim 3 wherein the material analyzed is an aqueousdispersion having organic components.

6. A method as in claim 3 wherein the catalyst bed comprises a noblemetal.

7. A method as in claim 5 including the additional step of cooling theeflluent gas from the heating zone prior to introducing the same intothe analyzer.

8. A method for determining the oxygen demand of a a material whichcomprises:

incorporating a reducing agent as a gas into a flowing feed gas streamcontaining carbon dioxide as essentially the sole oxidant,

flowing the feed gas stream containing the reducing agent and carbondioxide into a combustion conduit having a heating zone at a temperatureof at least 500 C. and through a catalyst bed in the heating zone of thecombustion conduit, said catalyst bed being effective to promote theequilibration of carbon dioxide with oxidizable components of thematerial to be analyzed,

inserting a predetermined quantity of the material to be analyzed intothe gas stream on the upstream side of the catalyst bed within theheated combustion conduit, and

sweeping the gaseous product formed through the catalyst bed andetfluent gas from the heating zone into an analyzer for quantitativelyindicating carbon monoxide.

9. A method as in claim 8 wherein the flowing feed gas stream isestablished at a constant and predetermined flow rate and the heatingzone of the combustion conduit is maintained at a temperature within therange from 500 to 1000 C.

10. A method for determining the oxygen demand of a material whichcomprises:

flowing a feed gas stream containing carbon dioxide as essentially thesole oxidant into carbon monoxidegenerating means whereby a small amountof carbon monoxide relative to the carbon dioxide is introduced into thefeed gas,

flowing the feed gas stream to a combustion conduit having a heatingzone at a temperature of at least 500 C. and through a catalyst bed inthe heating zone of the combustion conduit, said catalyst bed beingeffective to promote the equilibration of carbon dioxide with oxidizablecomponents of the material to be analyzed,

inserting a predetermined quantity of the material to be analyzed intothe feed gas stream on the upstream side of the catalyst bed within thecombustion conduit, and

sweeping the gaseous product formed through the catalyst bed andeflluent gas from the heating zone into an analyzer for quantitativelyindicating the amount of carbon monoxide in the eflluent gas stream.

11. A method as in claim 10 wherein the flowing feed gas streamcontaining carbon dioxide as essentially the sole oxidant is establishedat a constant and predetermined flow rate from a uniform supply ofcarbon dioxide-containing gas and the carbon monoxide-generating meanscomprises a bed of carbon heated at a temperature in the range fromabout 300 to 600 C.

12. A method as in claim 11 wherein the material analyzed is an aqueousdispersion containing organic components.

13. A method as in claim 11 wherein the material analyzed is an aqueousdispersion containing organic components and including the additionalstep of cooling the efliuent gas from the heating zone prior tointroducing the same into the analyzer.

14. A method as in claim 11 wherein the analyzer continuously monitorsthe carbon monoxide content of the efiluent gas from the heating zoneand produces an electrical signal relative thereto.

15. A method as in claim 14 including the additional step of calibratingthe electrical signal indicating increased carbon monoxide, to determinethe total oxygen demand (TOD) of the material analyzed.

16. A method as in claim 14 including the additional step of calibratingthe electrical signal indicating decreased carbon monoxide, to determinethe oxidizing capacity of the sample analyzed.

17. An apparatus for determining the total oxygen demand of a materialwhich apparatus comprises:

(1) flow control means for maintaining a confined continuous feed gasstream containing carbon dioxide from a pressurized source of supply ata constant and predetermined flow rate;

(2) a combustion conduit having an inlet and an outlet, said combustionconduit being coupled at the inlet to the flow control means for thefeed gas and having a heating zone in which there is a gas-permeable,catalyst body of a material which promotes the equilibration of carbondioxide with the oxidizable components of the material analyzed at anelevated temperature, said combustion conduit being adapted at the gasinlet end thereof to receive sample injection means for inserting apredetermined amount of the material to be analyzed into the heatingzone upstream from the catalyst bed,

(a) heating means in heat exchange relationship with the combustionconduit for maintaining the heating zone thereof at a controlledtemperature within the range of about 500 to about 1000 C.,

(3) carbon monoxide-detection means coupled to the combustion conduitfor quantitatively indicating the carbon monoxide content in a gasstream;

said gas flow control means, combustion conduit, and carbonmonoxide-detection means being coupled in the specified order bysuitable gas conduits to provide a continuous gas stream.

18. An apparatus as in claim 17 wherein the carbon monoxide detectionmeans is an infrared analyzer which produces an electrical signalrelative to the carbon monoxide content of a gas stream.

19. An apparatus as in claim 17 and including means for injecting amaterial to be analyzed into the combustion conduit upstream from thecatalyst bed in operative association with the combustion conduit andmeans for returning gas from the carbon monoxide detection means to agas blanket jacket surrounding the injection port to the combustionconduit.

20. An apparatus as in claim 17 and including in addition, means withinthe feed gas train between the flow control means and combustion conduitfor generating carbon monoxide.

21. An apparatus as in claim 20 wherein the carbon monoxide-generatingmeans is a bed of gas-permeable 13 i 14 carbon confined within a heatingzone in heat exchange 3,224,837 12/1965 Moyat 23230 relationship withmeans for maintaining the heating zone 3,249,403 5/ 1966 Bochinski et al23-253 at a temperature within the range from 450 to 650 C.

MORRIS O. WOLK, Primary Examiner.

Refe'ences Cited 5 R. E. SERWIN, Assistant Examiner. UNITED STATESPATENTS U 8 cl XR 3,205,045 9/1965 Von Lossberg 23253 23-232, 253, 254

1. A METHOD FOR DETERMINING THE OXYGEN DEMAND OF A MATERIAL WHICHCOMPRISES: FLOWING A FEED GAS STREAM CONTAINING CARBON DIOXIDE ASESSENTIALLY THE SOLE OXIDANT INTO A COMBUSTION CONDUIT HAVING A HEATINGZONE AT A TEMPERATURE OF AT LEAST 500*C. AND THROUGH A CATALYST BED INTHE HEATING ZONE OF THE COMBUSTION CONDUIT, SAID CATALYST BED BEINGEFFECTIVE TO PROMOTE THE EQUILIBRATION OF CARBON DIOXIDE WITH OXIDIZABLECOMPONENTS OF THE MATERIAL TO BE ANALYZED, INSERTING A PREDETERMINEDQUANITY OF THE MATERIAL TO BE ANALYZED INTO THE GAS STREAM ON THEUPSTREAM SIDE OF THE CATALYST BED WITHIN THE COMBUSTION CONDUIT, ANDSWEEPING GASEOUS PRODUCT FORMED THROUGH THE CATALYST BED AND EFFLUENTGAS FROM THE HEATING ZONE INTO AN ANALYZER FOR QUANTITATIVELY INDICATINGCARBON MONOXIDE.